1. Field of the Invention
The present invention relates to an apparatus and method for use in a wellbore. In addition, the invention relates to a downhole tool for determining the location and nature of an obstruction in a wellbore. More particularly still, the invention relates to a downhole tool for locating the point at which a tubular such as a drill string is stuck in a hollow tubular or a wellbore.
2. Description of the Related Art
Wellbores are typically formed by boring a hole into the earth through use of a drill bit disposed at the end of a tubular string. Most commonly, the tubular string is a series of threadedly connected drill collars. Weight is applied to the drill string while the drill bit is rotated. Fluids are then circulated through a bore within the drill string, through the drill bit, and then back up the annular region formed between the drill string and the surrounding earth formation. The circulation of fluid in this manner serves to clear the bottom of the hole of cuttings, serves to cool the bit, and also serves to circulate the cuttings back up to the surface for retrieval and inspection.
With today's wells, it is not unusual for a wellbore to be completed in excess of ten thousand feet. The upper portion of the wellbore is lined with a string of surface casing, while intermediate portions of the wellbore are lined with liner strings. The lowest portion of the wellbore remains open to the surrounding earth during drilling. As the well is drilled to new depths, the drill string becomes increasingly longer. Because the wells are often non-vertical or diverted, a somewhat tortured path can be formed leading to the bottom of the wellbore where new drilling takes place. Because of the non-linear path through the wellbore, the drill string can become bound or other wise stuck in the wellbore as it moves axially or rotationally. In addition, the process of circulating fluids up the annulus within the earth formation can cause subterranean rock to cave into the bore and encase the drill string. All drilling operations must be stopped and valuable rig time lost while the pipe is retrieved.
Because of the length of the drill string and the difficulty in releasing stuck pipe, it is useful to know the point at which one tubular is stuck within another tubular or within a wellbore. The point above the stuck point is known as the “free point.” It is possible to estimate the free point from the surface. This is based upon the principle that the length of the tubular will increase linearly when a tensile force within a given range is applied. The total length of tubular in the wellbore is known to the operator. In addition, various mechanical properties of the pipe, such as yield strength and thickness, are also known. The operator can then calculate a theoretical extent of pipe elongation when a certain amount of tensile force is applied. The theoretical length is based on the assumption that the applied force is acting on the entire length of the tubular.
The known tensile force is next applied to the tubular. The actual length of elongation of the pipe is then measured at the surface of the well. The actual length of elongation is compared with the total theoretical length of elongation. By comparing the measured elongation to the theoretical elongation, the operator can estimate the sticking point of the tubular. For example, if the measured elongation is fifty percent of the theoretical elongation, then it is estimated that the tubular is stuck at a point that is approximately one half of the length of the tubular from the surface. Such knowledge makes it possible to locate tools or other items above, adjacent, or below the point at which the tubular is expected to be stuck.
It is desirable for the operator to obtain a more precise determination of the stuck point for a string of pipe. To do this, the operator may employ a tool known as a “free point tool.” The prior art includes a variety of free point apparatuses and methods for ascertaining the point at which a tubular is stuck.
One common technique involves the use of a tool that has either one or two anchors for attaching to the inner wall of the drill pipe. The tool is lowered down the bore of the drilling pipe, and attached at a point to the pipe. The tool utilizes a pair of relatively movable sensor members to determine if relative movement occurred. The tool is located within the tubular at a point where the stuck point is estimated. The tool is then anchored to the tubular at each end of the free point tool, and a known tensile force (or torsional force) is applied within the string. Typically, the force is applied from the surface. If the portion of the pipe between the anchored ends of the free point tool is elongated when a tensile force is applied (or twisted when a torsional force is applied), it is known that at least a portion of the free point tool is above the sticking point. If the free point tool does not record any elongation when a tensile force is applied (or twisting when a torsional force is applied), it is known that the free point tool is completely below the sticking point. The free point tool may be incrementally relocated within the drill pipe, and the one or more anchor members reattached to the drill pipe. By anchoring the free point tool within the stuck tubular and measuring the response in different locations to a force applied at the surface, the location of the sticking point may be accurately determined.
Mechanical free point tools of this type are considered reliable; however, they suffer from certain disadvantages. For example, mechanical transducer free point tools rely upon moving parts. It is desirable to have a free point tool that contains few or no moving parts. In addition, mechanical free point tools are considered slow to operate. In this respect, the sequential attachment and detachment of the free point tool to the drill string requires time. Those familiar with the drilling industry understand that the operation of a drilling rig, particularly those located offshore, is very expensive.
Other tools have been developed which include means for measuring the magnetic permeability of the pipe. In this regard, one known characteristic of ferromagnetic pipe is that the magnetic permeability of the material changes as a function of stresses in the material. This principle allows for the detection of changes in magnetic flux rather than mechanical movement. The operator maintains constant tension in the stuck pipe from the surface, and allows the magnetic permeability tool sensor to operate while the tool is being moved through a selected section of drill pipe. The operator maintains data that correlates changes in magnetic flux to depth of the tool. This may prove to be a faster procedure than free point tools that rely upon sequential mechanical anchoring to the drill string. However, the operation of such a tool remains expensive, as it requires that an electrical wireline be provided for running into the wellbore.
A need therefore exists for a free point tool that can be quickly run into a wellbore on a more economical basis. A need alternatively exists for a free point logging tool that employs digital telemetry memory technology to store detected information downhole for quick retrieval and subsequent analysis. Still further, a need exists for a free point tool that combines features of an acoustic stuck pipe logging tool (which graphically presents information as to the stuck condition of a pipe), with a free point sensor in one logging string package.